Calorimetry and Enthalpy
Chapter 5.2
Heat Capacity
• Specific heat capacity (c) is the quantity of
thermal energy required to raise the temperature
of 1g of a substance by 1⁰C
• The units for specific heat capacity are J/(g•⁰C)
The water has a higher
specific heat capacity so
it requires more
thermal energy to raise
its temperature
The sand has a lower
specific heat capacity so
it requires a less
thermal energy to raise
its temperature
Heat Capacity
• Specific heat capacity values can be looked up in tables
• There is one in your textbook on page 292
Calorimetry and Thermal Energy Transfer
• Calorimetry is the experimental process of measuring the
thermal energy change in a chemical or physical change
• A calorimeter is a device that is used to measure thermal
energy changes in a chemical or physical change
Calorimetry Calculations
• To simplify our calculations we will
make three assumptions:
1.
2.
3.
Any thermal energy transferred from
the calorimeter to the outside
environment is negligible
Any thermal energy absorbed by the
calorimeter itself is negligible
All dilute, aqueous solutions have
the same density (1.00g/mL) and
specific heat capacity (4.18 J/(g•⁰C))
as water
Calorimetry Calculations
• We will use the following equation:
q is the total amount
of thermal energy
absorbed or released
by a chemical system
in joules
∆T is the temperature change
experienced by the substance
as it warms or cools in ⁰C
q = mc∆T
m is the mass of the
substance in grams
note: ∆T = Tfinal - Tinitial
c is the specific heat
capacity of the
substance in J/(g•⁰C)
The value of q has two parts:
q = - 237J
1. The magnitude tells us how much energy is involved
2. The sign tells us the direction of energy transfer
Because of the law of conservation
of energy, the total thermal energy
of the system and its surroundings
remains constant:
qsystem + qsurroundings = 0
qsystem = - qsurroundings
The chemical system undergoes either a physical change or
a chemical change
Energy is either absorbed from, or released to, the
surroundings (the water in the calorimeter)
An increase in the temperature of the water indicates an
exothermic reaction, whereas a decrease in the
temperature of the water indicates an endothermic
reaction
Practice Problem 1
• 600mL of water in an electric kettle is heated from
20⁰C to 85⁰C to make a cup of tea. How much
thermal energy is absorbed by the water?
Practice Problem 2
• The temperature of an aluminum fence post at
5pm is 20⁰C. The same fence post has a
temperature of 6⁰C by 11pm. If the fence post
releases 315kJ of thermal energy to its
surroundings, what is the mass of the fence post?
Practice Problem 3
• 50.00mL of aqueous copper (II) sulfate reacts with
50.00mL of aqueous sodium hydroxide in a
calorimeter. The initial temperature of both
solutions is 21.40⁰C and the highest temperature
reached in the calorimeter is 24.60⁰C. Determine
the quantity of thermal energy transferred by the
reaction to the water, and state whether the
reaction was endothermic or exothermic.
Enthalpy Change
• Enthalpy (H) is the total amount of thermal
energy in a substance
• Enthalpy Change (ΔH) is the energy released
to or absorbed from the surroundings during a
chemical or physical change
Enthalpy Change Can be Measured
Enthalpy change can be measured using
calorimetry data
As long as pressure is kept constant, the
enthalpy change of a chemical system is equal
to the flow of thermal energy into or out of
the system
ΔHsystem = |qsystem|
Calculating the enthalpy change can give us
information about type of reaction
ΔH = Hproducts – Hreactants
If ΔH > 0, the reaction is endothermic
If ΔH < 0, the reaction is exothermic
Molar Enthalpy Change
• Molar enthlapy change (ΔHr) is the enthalpy change
associated with a physical, chemical, or nuclear change
involving one mole of a substance
• The units for molar enthalpy change are J/mol
Molar Enthalpy Change
Many molar enthalpy changes have been carefully measured by scientists and the
values are published in tables like the one below and can easily be looked up
Using Molar Enthalpy Change
• Molar enthlapy values (ΔHr) can help us calculate an
enthalpy change (ΔH)
• To calculate an enthalpy change (ΔH) for some
amount of substance other than one mol, we need
to look up the molar enthalpy value (ΔHr) and then
multiply it by the number of moles (n) using the
formula below
ΔH = nΔHr
Practice Problem 4
• A common refrigerant (Freon-12 with molar mass
120.91g/mol) is alternately vaporized in tubes inside a
refrigerator, absorbing heat, and condensing in tubes
outside the refrigerator, releasing heat. This results in
energy being transferred from the inside to the outside
of the refrigerator. The molar enthalpy of vaporization
for the refrigerant is 34.99kJ/mol. If 500.0g of the
refrigerant is vaporized, what is the expected enthalpy
change?
Representing Enthalpy Changes
• There are four ways to represent enthalpy
changes:
1.
2.
3.
4.
Thermochemical equations with energy terms
Thermochemical equations with ΔH values
Molar Enthalpies
Potential Energy Diagrams
HOMEWORK
Required Reading:
p. 292-306
(remember to supplement your notes!)
Questions:
p. 297 #1-3
p. 301 #1-4
p. 304 #1-4
p. 306 #1-7